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July 1, 2002
Volume 80, Number 26
CENEAR 80 26 p. 7
ISSN 0009-2347


Automated method builds precision parts from tailor-made colloidal inks

Imagine building a staircase using only one tool: a spray gun that squirts out a stream of concrete that spontaneously assumes and holds the required shape. A construction technique similar to this fanciful one has just been demonstrated by materials scientists.

Using colloidal gels--inks--as construction materials, researchers at the University of Illinois, Urbana-Champaign, and Sandia National Laboratory in Albuquerque have devised a procedure for building intricate three-dimensional structures with micrometer-sized features and overall dimensions of a few millimeters [Langmuir, published online June 14,]. The new method may lead to applications in advanced ceramics, photonic materials, catalyst supports, and other areas.

INTRICATE Measuring just a few millimeters across, this complex structure was made from colloidal inks using an automated layer-by-layer deposition process.
"Our goal is to make designer materials that can't be made by conventional forming techniques," says associate professor of materials science and engineering Jennifer A. Lewis, who led the study. The approach is based on robocasting--a process developed by Sandia staff scientist and coauthor Joseph Cesarano III, in which a computer-controlled robotic arm delivers material through a fine nozzle onto a moving platform to build structures one layer at a time.

The trick is coming up with just the right materials to squirt through the nozzles. "Traditional inks, like the ones used in ink-jet printers, are very fluid," Lewis points out. "They have low concentrations of solids, and on contact, they spread and wet the surface." For robotic fabrication, the inks need to be highly concentrated yet fluid enough to flow through the nozzles and then set immediately so that the structures won't buckle or sag. The materials must also resist shrinking, which commonly occurs as gels dry.

Working with graduate student James E. Smay, now an assistant professor of chemical engineering at Oklahoma State University, Stillwater, Lewis and Cesarano used polyelectrolytes to tune the strength of forces acting between colloid particles--to stabilize the dispersions--and then gelled the fluids by changing the pH or ionic strength. In this way, the group prepared inks of silica, alumina, lead zirconate titanate, and other materials with the required properties.

Inks can be made from nearly any particulate material that can be suspended in solution, Lewis explains, so long as the forces between particles can be tuned to yield the desired viscoelastic response. For example, the group has recently prepared gels made of hydroxyapatite, a principal component of bone, offering potential for designing model scaffolds for tissue engineering.


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Copyright © 2002 American Chemical Society

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